Reluctance electric machines, such as motors and generators, typically include a stator that is mounted inside a machine housing and a rotor that is supported for rotation relative to the stator. Reluctance electric machines produce torque as a result of the rotor tending to rotate to a position that minimizes the reluctance (or maximizes the inductance) of the magnetic circuit. The reluctance is minimized (and the inductance is maximized) when the salient rotor poles are aligned with the energized salient stator poles. A drive circuit generates a set of stator winding currents that are output to the stator pole windings and that create a magnetic field. The rotor rotates in response to the magnetic field. In synchronous reluctance electric machines, the windings are energized at a controlled frequency. In switched reluctance electric machines, the drive circuit and/or transducers detect the angular position of the rotor. The drive circuit energizes the stator windings as a function of the sensed rotor position. The design and operation of switched reluctance electric machines is known in the art and is discussed in T. J. E. Miller, “Switched Reluctance Motors and Their Control”, Magna Physics Publishing and Clarendon Press, Oxford, 1993, which is hereby incorporated by reference.
Conventional switched reluctance electric machines generally include a stator with a solid stator core or a laminated stator. The laminated stator includes a plurality of circular stator plates that are punched from a magnetically conducting material. The stator includes pairs of diametrically opposed stator poles that project radially inward. The rotor also includes pairs of diametrically opposed rotor poles. Windings or coils are typically disposed about the stator poles. The windings that are wound around any two diametrically opposed stator poles are connected in series or in parallel to define a machine phase or a phase coil.
By passing current through the phase coil, magnetic fields are established about the stator poles and torque is produced as the energized phase coil attracts a pair of rotor poles into alignment. The current in the phase coils is generated in a predetermined sequence to create the magnetic field that produces continuous rotating torque on the rotor. The period during which current is provided to the phase coil is known as the active stage. At a predetermined point, either as the rotor poles become aligned with the stator poles or at some point prior thereto, the current in the phase coil is commutated to prevent braking torque from acting on the rotor poles. Once the commutation point is reached, the current is switched to another phase coil. During the inactive stage, the current is allowed to dissipate from the phase coil.
In order to maintain torque on the rotor, it is important to maintain the proper relationship between the position of the rotor and the active stage of each machine phase. If the active stage is initiated and/or commutated too early or too late with respect to the position of the rotor, the torque on the rotor will vary and/or the machine will not operate at optimum efficiency.
The drive circuits of conventional switched reluctance electric machines control the current in the phase coils. The drive circuits maintain the proper relationship between the active stage of the machine phases and the position of the rotor by continuously sensing rotor position. There are two distinct approaches for detecting the angular position of the rotor. In a “sensed” approach, an external physical sensor senses the angular position of the rotor. For example, a rotor position transducer (RPT) with a hall effect sensor or an optical sensor physically senses the angular position of the rotor. In a “sensorless” approach, electronics that are associated with the drive circuit derive the angular rotor position without an external physical sensor.
There are many problems that are associated with switched reluctance electric machines that employ the sensed approach. The RPT typically includes a sensor board with one or more sensors and a shutter that is coupled to and rotates with the shaft of the rotor. The shutter includes a plurality of shutter teeth that pass through optical sensors as the rotor rotates. Because the angular rotor position is critical to proper operation, sophisticated alignment techniques are used to ensure that the sensor board of the RPT is properly positioned with respect to the housing and the stator. Misalignment of the sensor board is known to degrade the performance of the electric machine. Unfortunately, utilization of these complex alignment techniques increases the manufacturing costs for switched reluctance electric machines equipped with RPTs.
The RPTs also increase the overall size of the switched reluctance electric machine, which can adversely impact machine and product packaging requirements. The costs of the RPTs often place switched reluctance electric machines at a competitive disadvantage in applications that are suitable for open loop induction motors that do not require RPTs. Another drawback with RPTs involves field servicing of the switched reluctance electric machines. Specifically, wear elements, such as the bearings, that are located within the enclosed rotor housing may need to be repaired or replaced. To reach the wear elements, an end shield must be removed from the housing. Because alignment of the sensor board is critical, replacement of the end shield often requires the use of complex realignment techniques. When the alignment techniques are improperly performed by the service technician, the sensor board is misaligned and the motor's performance is adversely impacted.
Various methods for dispensing with the RPT have been proposed. Several of these are reviewed in “Sensorless Methods for Determining the Rotor Position of Switched Reluctance Motors” by W F Ray and I H Al-Bahadly, published in the Proceedings of The European Power Electronics Conference, Brighton, UK, 13-16 September 1993. Vol. 6, pp 7-13, hereby incorporated by reference. Many of these methods proposed for the rotor position estimation use the measurement of phase flux-linkage (i.e. the integral of applied voltage with respect to time) and current in one or more phases. Position is calculated using knowledge of the variation in inductance of the machine as a function of angle and current. The storage of this data involves the use of a large memory array and/or additional system overheads for interpolation of data between stored points.
In U.S. Pat. No. 5,777,416 to Kolomeitsev, U.S. Pat. No. 6,011,368 to Kalpathi et al, and U.S. Pat. No. 6,107,772 to Liu et al., which are incorporated by reference, a drive circuit measures the rise time of current in a stator winding between two predetermined current levels. The drive circuit calculates the inductance of the phase coil from the current rise time. The drive circuit estimates the angular position of the rotor from the inductance of the phase coil. The drive circuit adjusts the active stage of the phase coil based on the rotor position. U.S. Pat. No. 5,982,117 to Taylor et al. and U.S. Pat. No. 5,883,485 to Mehlhorn, which are incorporated by reference, likewise monitor current in unenergized windings to determine the inductance of the phase coil and the position of the rotor. In U.S. Pat. No. 4,772,839 to MacMinn, which is incorporated by reference, a drive circuit simultaneously measures changes in the current in two unexcited phases. The drive circuit derives rotor position estimates for each phase and combines the rotor position estimates into a combined rotor position estimate.
Some methods make use of this data at low speeds where “chopping” current control is the dominant control strategy for varying the developed torque. These methods usually employ diagnostic energization pulses in non torque-productive phases (i.e. those phases which are not energized directly from the power supply at a particular moment). A method suited to low-speed operation is proposed by N M Mvungi and J M Stephenson in “Accurate Sensorless Rotor Position Detection in a S R Motor”, published in Proceedings of the European Power Electronics Conference, Firenze, Italy, 1991, Vol. 1, pp 390-393, which is hereby incorporated by reference.
In U.S. Pat. No. 4,959,596 to MacMinn, et al., which is incorporated by reference, a drive circuit employs a phase inductance sensing technique to indirectly estimate rotor position. Voltage sensing pulses are output to an unexcited phase. The voltage sensing pulses cause a change in phase current that is inversely proportional to the instantaneous phase inductance. Commutation time is determined by comparing the change in phase current to a threshold current. U.S. Pat. No. 5,589,518 to Vitunic which is incorporated by reference, also discloses a drive circuit that employs diagnostic pulses.
Other methods employ the “single-pulse” mode of energization at higher speeds. The current and inductance waveforms, over a phase inductance period, are mirror images of the monitoring waveforms. These methods monitor the operating voltages and currents of an active phase without interfering with normal operation. A typical higher speed method is described in U.S. Pat. No. 5,173,650 to Hedlund, which is hereby incorporated by reference.
Both the chopping and single-pulse modes described above are normally used when the converter applies a fixed value of supply voltage to the phase windings. A further control strategy is the pulse width modulated (PWM) mode, where one or more switches are switched rapidly to effectively produce a supply voltage that is proportional to the duty cycle of the PWM waveform. This allows the use of single-pulse current waveforms at much lower speeds than would be possible on the full supply voltage. The current waveform is made up of a large number of segments, corresponding to the current carried by the switches and diodes respectfully.
Having to store a two-dimensional array of machine data in order to operate without a position sensor is an obvious disadvantage. Alternative methods have been proposed, which avoid the need for the majority of angularly referenced information and instead store data at one angle only. One such method described in U.S. Pat. No. 5,467,025 to Ray which is hereby incorporated by reference. This method senses the phase flux-linkage and current at a predefined angle by adjusting the diagnostic point via the calculated deviation away from a desired point. Two one-dimensional tables are stored in the preferred embodiment, one of flux-linkage versus current at a referenced rotor angle and another of the differential of flux-linkage with respect to rotor angle versus current. By monitoring phase voltage and current, the deviation away from a predicted angle can be assessed, with the aid of the look-up tables, and system operation can be adjusted accordingly. However, such methods, although reducing the amount of information which has to be stored, still have to detect or compute the flux-linkage at a specific rotor angle and may be sensitive to repeatability or manufacturing tolerances in the machine.
A similar approach is disclosed in U.S. Pat. No. 5,793,179 to Watkins, hereby incorporated by reference, where the arrival of the rotor at the peak of the inductance profile is predicted and the system is then put into a freewheeling mode, during which the gradient of the current is measured. While this method works well in the absence of noise, it is not robust enough to disregard false readings produced by noise. Though the current waveform may be relatively immune to induced noise, a drive that uses a PWM voltage supply generates a noisy current waveform. The method disclosed by Watkins '179 also must be used with a converter circuit that is capable of freewheeling.
Other attempts to overcome these deficiencies are described in “A New Rotor Position Estimation Method for Switched Reluctance Motors using PWM Voltage Control”, by Gallegos-Lopez, G. Kjaer, P C & Miller, T J E, in Proc EPE'97, 7th European Conf on Power Electronics and Applications, 8-10 Sep. 1997, Trondheim, Norway, Vol. 3, pp 580-585, hereby incorporated by reference. This method continuously samples the current waveform and attempts to detect the change in gradient that is produced by the start of pole overlap and the consequent sudden rise in inductance of the phase. The basic method described by Gallegos-Lopez et al involves detecting the point of pole overlap for monitoring (or pole separation for generating) by detecting the point where the rate of change of the current waveform, with respect to time, is zero. The detector includes a differentiator, a comparator and a single shot multivibrator. The differentiator differentiates the current signal so that at the point of zero di/dt the differentiator output is zero. The comparator detects this zero output and flips state. The system does not require either stored magnetization data or an interval of freewheeling. The system does current feedback and does not work reliably in the presence of noise. Improvements to this system include sampling and storage of over several intervals and interpolation to reduce the effects of false detection caused by noise.
When sensing the angular rotor position using the sensorless approach, variations in the electrical characteristics of the individual stator pole windings can adversely impact the ability of the sensorless drive circuits to correctly derive the angular position of the rotor. Most of the sensorless approaches measure the resistance and/or inductance of the windings. If the resistance and/or inductance varies from one stator winding to another, the sensorless drive circuit may incorrectly derive the angular position of the rotor. This problem is made worse if the windings on the stator poles creep or move over time. When this occurs, the cross section of the stator windings changes, which changes the inductance and resistance of the stator pole winding.
There are several conventional methods for placing the winding wire on the stator of a switched reluctance electric machine. The winding wire can be initially wound and transferred onto the stator poles. Transfer winding tends to leave excess winding wire or loops around axial ends of the stator poles. Transfer winding can typically utilize approximately 60-65% of available stator slot area. Needle winding employs a needle that winds the wire directly on the stator poles. The needle, however, takes up some of the stator slot area, which reduces slot fill to approximately 50%. The positioning of winding wire on the stator poles using these methods varies from one stator pole to the next. Winding creep and other assembly variations also impact the inductance and resistance of the winding wire over time, which makes it difficult to accurately perform “sensorless” control due to the non-conformity of the salient stator poles.
While the design of switched reluctance electric machines is relatively mature, there are several areas requiring improvement. Specifically, it is desirable to improve the uniformity of the electrical characteristics of the stator of switched reluctance electric machines. It is also desirable to eliminate the need for RPTs in switched reluctance electric machines to decrease the cost and to improve both durability and serviceability.